A rolling bearing according to this invention is incorporated into a rotation supporting section which is installed outdoors and may be driven at low temperatures, such as for example, a propeller shaft of a vehicle, or a rotation shaft of an electric motor, to thereby prevent the occurrence of harmful vibration or noise even at low temperatures.
In order to dampingly support for example, a middle portion of a propeller shaft of a vehicle under a floor of the vehicle, a rotation supporting unit 1 as shown in FIG. 1 is used. This rotation supporting unit 1 supports a rolling bearing 3 via a buffer material 4 and a housing 5, on the inner diameter side of a support bracket 2. Of these, the buffer material 4 is formed from a material having a large internal loss, such as rubber, so as to be freely displaceable in the radial direction (vertical direction in FIG. 1) and in thee axial direction (lateral direction in FIG. 1). The housing 5 is formed by fitting and combining elements 6a, 6b respectively formed in a cylindrical shape, for fittingly supporting an outer ring 7 of the rolling bearing 3 on the inside.
The rolling bearing 3, being a deep groove ball bearing, comprises an inner ring 9 having an inner ring raceway 8 on an outer peripheral surface thereof, an outer ring 7 having an outer ring raceway 10 on an inner peripheral surface thereof, and a plurality of rolling bodies (balls) 11, 11 arranged rollably between the inner ring raceway 8 and the outer ring raceway 10. The rolling bodies 11, 11 are rollably held by a retainer (not shown), respectively. Also, grease is filled into a space 12 existing between the outer peripheral surface of the inner ring 9 and the inner peripheral surface of the outer ring 7, in which the rolling bodies 11, 11 are arranged, and openings on the opposite ends of this space 12 are respectively sealed by ring-shaped seal rings 13, 13. Such a rolling bearing 3 constitutes the rotation supporting unit 1 described above, in such a manner that the outer ring 7 is supported under the floor of the vehicle via the housing 5 and the buffer material 4, and the inner ring 9 on the outside is fittingly secured to the middle portion of the propeller shaft 14.
The rotation supporting unit 1 for a propeller-shaft described above, or a rotation supporting apparatus incorporated in an electric motor installed outside, is used in a low temperature environment in the winter season. In the case of the rotation supporting unit 1 used under such a low temperature environment and lubricated by grease, when the temperature in the portion of the rolling bearing 3 is still low just after startup, abnormal vibration often occurs, and it is known that when the vibration is conspicuous, offensive noise to the ear occurs due to this abnormal vibration. In particular, it is known that under an environment of xe2x88x9210xc2x0 C. or less, the offensive noise occurs conspicuously.
The mechanism in which such a noise occurs was elucidated in a paper xe2x80x9cResearch Regarding Abnormal Vibration of Ball Bearingsxe2x80x9d described in Nihon Machinery Society Paper, Vol. 63, No. 616 (Chapter C), pp 250xcx9c256, issued in December 1997. According to this paper, the above described mechanism is such that self-excited vibration of the rolling body becomes a cause of the above described abnormal vibration.
For example, if a rotation supporting unit 1 for supporting the middle portion of a propeller shaft 14 as shown in FIG. 1 is considered, when an axial force is applied to the rolling bearing 3 due for example to a change in the operating conditions, external disturbances, or friction in a joint portion disposed at the end of the propeller shaft, axial slippage occurs in each abutting portion between the rolling surface of the respective rolling bodies 11, 11 and the inner ring race 8 and the outer ring race 10. This generates a shear rate in the oil film of the grease intervening in each abutting portion. When this shear rate exceeds a certain value, the shear stress decreases, and the grease acts as a negative resistance. That is to say, there is a relationship expressed by an equation of equilibrium in the lubricant film: dp/dx=dxcfx84/dy, between the pressure p of the oil film in the grease and the shear stress xcfx84 of the oil film. As is obvious from this relation, as the shear stress xcfx84 of the oil film decreases, the pressure p of the oil film also decreases, to generate the self-excited vibration of the respective rolling bodies 11, 11 whose rolling surface abuts against the oil film. It is made clear in the paper xe2x80x9cSimulation Regarding Nonlinear Vibration of Ball Bearingsxe2x80x9d described in Nihon Machinery Society Lecture Paper, No. 985-2, pp 269, issued in October 1998, that the frequency of such self-excited vibration becomes a multiple of the rotational frequency.
When the self-excited vibration occurs in the respective rolling bodies 11, 11 with the above described mechanism, the film thickness of the grease existing in the inner ring raceway 8, the outer ring raceway 10 and the rolling surface portion of the respective rolling bodies 11, 11 becomes uneven over the circumference, depending on the traveling tracks based on the rotation movement and revolution movement of the respective rolling bodies 11, 11. In other words, an undulation (bank of grease) is formed by the grease on the surface portion of the inner ring raceway 8 and the outer ring raceway 10 and on the rolling surface portion of the respective rolling bodies 11, 11, respectively. The banks of grease formed in this manner cause radial and axial vibrations, in the same manner as undulations originally existing on the inner ring raceway 8, the outer ring raceway 10 or the rolling surface of the respective rolling bodies 11, 11.
If the frequency of the self-excited vibration of the rolling bodies coincides with the vibration frequency resulting from the banks of grease, vibration is promoted. Moreover, if the frequency of the self-excited vibration of the rolling bodies and the vibration frequency resulting from the banks coincide with the vibration frequency resulting from undulations originally existing (not the banks of grease, but originally existing) on the inner ring raceway 8, the outer ring raceway 10 or the rolling surface of the respective rolling bodies 11, 11, vibration is further promoted to become large. Vibration which has grown in such a manner resonates with a member circumjacent to the rolling bearing 3, thereby resulting in offensive noise to the ear.
Abnormal vibration and noise described above occurs in the first place, based on the self-excited vibration of the rolling bodies and this self-excited vibration grows due to the banks of grease. Accordingly, in order to eliminate the above described banks of grease or to reduce the strength thereof so that the self-excited vibration does not grow to an abnormal vibration, grease having a low viscosity may be used. However, even with such a measure, it is difficult to obtain a sufficient effect at extremely low temperatures, as low as xe2x88x9210xc2x0 C. Moreover, if grease having such a low viscosity that abnormal vibration does not occur even at such an extremely low temperature is used, the grease tends to leak through the above described respective seal rings 13, 13. Furthermore, grease having a low viscosity has a weak oil film holding power at the rolling contact portion, and in many cases, it is not always satisfactory from the standpoint of lubricating ability. As a result, when grease having a low viscosity is used, it becomes difficult to favorably maintain the lubricating condition of the rolling bearing 3 for a long period of time.
The present invention addresses the above situation, and realizes a rolling bearing which hinders growth of the self-excited vibration of the rolling bodies to an abnormal vibration, without using a special grease, and which has sufficient durability, and which does not generate abnormal vibration or noise, even if it is used at low temperatures.
All the rolling bearings of the present invention comprise a first race having a first raceway, a second race having a second raceway, and Z rolling bodies rollably disposed between the first raceway and the second raceway, as with the aforesaid conventional rolling bearing.
In particular, in the rolling bearing of the present invention, when assumed that n is a positive integer, the frequencies of the vibration generated due to the circumferential undulations of (nZ) waves and (nZxc2x11) waves existing on the surface of the first and second raceways, and the undulation of (2n) waves existing on the rolling surface of the respective rolling bodies, are controlled by the relation with the frequencies in other parts.
At first, in the case of the rolling bearing according to claim 1, the frequencies of the vibration generated due to the above described undulations does not coincide with either the rotational frequency itself of the rotation member supported by the rolling bearing, or a frequency component which is a multiple of the rotational frequency.
Also, in the case of the rolling bearing according to claim 2, a plurality of kinds of vibration frequencies generated due to the above described undulations do not coincide with each other, in the natural frequency domain of a rotation system which is a rotation supporting portion constituted by incorporating the rolling bearing.
In the case of the rolling bearing of the present invention constructed as described above, banks of grease are formed due to the self-excited vibration of the rolling bodies, but this vibration is not susceptible to growth even if vibration occurs due to the banks of grease. As a result, harmful abnormal vibration and offensive noise to the ear are unlikely to occur. The reason for this will be described below.
At first, a description is given of the reason why the vibration frequencies controlled by the relation with the frequencies in other parts, are limited to one due to the undulations of (nZ) waves and (nZxc2x11) waves existing on the raceway surface, and the undulation of (2n) waves existing on the rolling surface of the respective rolling bodies. Here, it is well known, as described for example in Japanese Unexamined Patent Publication No. Toku Kai Hei 8-247153 or the like, that the undulations existing on the raceway surface and the rolling surface exist in a plurality of kinds in number, even when seen on the same surface.
It is also well known, as described in Japanese Unexamined Patent Publication No. Toku Kai Hei 8-247153, that if it is assumed that the number of the rolling bodies is Z, and n is a positive integer, then with regard to the undulations existing on the surface of the first and second raceways, the circumferential undulations of (nZ) waves and (nZxc2x11) waves cause larger vibration compared to the undulations of other numbers of waves. Moreover, with regard to undulations existing on the rolling surface of the respective rolling bodies, the undulation of the (2n) waves cause large vibration. This is because since the undulation of the (2n) waves is formed such that peaks and valleys in the undulation exist in the diametrically opposite positions of the rolling surface, variations in the diameter of the rolling surface due to the rotation of the rolling bodies, in other words, variations in the gap between the first and second raceways described above, which put the rolling surface therebetween, increase. Therefore, the undulations regarding the controlled vibration frequency are respectively limited to the (nZ) waves and the (nZxc2x11) waves with regard to the raceway surface and the (2n) waves with regard to the rolling surface.
On the assumption described above, the reason why with the rolling bearing of the present invention, harmful abnormal vibration or offensive noise to the ear is unlikely to occur will be described. At first, the vibration frequencies due to the undulations of the number of waves which tend to result in large vibration as described above, do not coincide with either the rotational frequency itself of the rotation member supported by the rolling bearing, or the frequency components which are a multiple of the rotational frequency, including a case of substantially coinciding therewith (to the extent of resonating, for example approaching within an error of 1 to 2%). As a result, even if banks of grease are formed due to the self-excited vibration of the rolling body, and vibration occurs due to the banks, the vibration due to the banks is not promoted by vibration due to the undulations existing on the surface of the first and second raceways and on the rolling surface, and hence the vibration does not grow.
That is to say, when the rolling bearing is used in a state that, for example, the inner ring, being the first race, rotates at fr (Hz), self-excited vibration occurs in the rolling bodies at a frequency of fxcfx89=nxc2x7fr, and based on this self-excited vibration, banks of grease having a shape corresponding to the frequency of fxcfx89=nxc2x7fr are formed on the inner ring raceway, being the first raceway, and the outer ring raceway, being the second raceway, and the rolling surface portion of the respective rolling bodies. On the contrary, if the vibration frequencies due to undulations existing on the surfaces of the first and second raceways and undulations existing on the rolling surface, shown in Table 1, deviate from the above described frequency, fxcfx89=nxc2x7fr (for example, by 1 to 2% or more, as described above), the undulations and the shape of the banks do not coincide with each other, thereby enabling prevention of growth of vibration due to the banks. Here, the degree of deviation in the frequencies so as not to cause resonance of the two vibrations more or less differs depending on various conditions such as the bearing size, but deviation of at least 1% is necessary. Moreover, if the deviation in the frequencies increases to 2% or more, resonance will not occur in almost all cases.
Wherein n: positive integer, Z: number of rolling bodies, fr: rotational speed of the inner ring (Hz), fc: rotational speed of a retainer {=revolving speed of the rolling bodies (Hz)}, fi=frxe2x88x92fc (Hz), fb: rotating frequency of the rolling bodies (Hz).
If the frequencies of the vibration due to undulations determined by the expression described in the above Table 1 are deviated from the frequency of fxcfx89=nxc2x7fr described above, banks of grease formed corresponding to the frequency of fxcfx89 are crushed between each rolling surface and the surface of the first and second raceways and collapse, as the respective rolling bodies carry out the rotation movement and the revolution movement. That is to say, if the above described both frequencies coincide with each other, banks of grease once formed further grow due to the vibration based on the undulations, and the vibration itself also grows, thereby resulting in the above described abnormal vibration and noise. On the contrary, if the above described both frequencies do not coincide with each other, the track of the respective rolling bodies which generates vibration due to the undulations does not coincide with the shape of the banks of grease. Hence, the respective rolling bodies crush this bank, to thereby prevent the vibration generated due to the self-excited vibration of the rolling bodies from growing. Rather, the bank of grease collapses to thereby absorb the energy of vibration due to the undulations, and hence alleviate the vibration due to the undulations.
Moreover, even in the case where a plurality of kinds of vibration frequencies generated due to the undulations existing on the surfaces of the first and second raceways and the rolling surface of the respective rolling bodies are made so as not to coincide with each other, in the natural frequency domain of the rotation member supported by the rolling bearing, the growth of the vibration can be suppressed, to thereby prevent the occurrence of the above described abnormal vibration and noise. That is to say, vibrations of different frequencies generated due to the undulations on the above described surfaces do not promote the growth each other, and vibration of at least any of the frequencies makes the banks of grease collapse, to thereby prevent the occurrence of the above described abnormal vibration and noise.